1. Technical Field
The present invention is directed to using a rotating sensor to scan the wall of the borehole. More particularly, the present invention is directed to using a rotating sensor, e.g., sonic or ultrasonic, to circumferentially scan the borehole wall.
2. Background Information
In order to perform measurements, e.g., to evaluate the quality of the cement bond with the casing of the well, various types of measurement apparatus have been developed.
For example, U.S. Pat. No. 4,524,433 to Broding, herein incorporated by reference, discloses an apparatus having at least one measurement sensor which points radially relative to the longitudinal axis of the apparatus and rotates to circumferentially scan the wall of the borehole. By combining this rotary motion of the sensor with the translation motion of the apparatus along the borehole, the wall of the borehole is scanned along a helical trajectory.
In measurement apparatus designed in this manner, there is no way of recalibrating the measurement sensor in situ as it scans the borehole. The operating conditions of the apparatus, e.g., temperature and pressure, vary along the borehole, and frequenttly result in changes in the measurements performed. In the absence of recalibration, this can lead to significant errors.
U.S. Pat. No. 4,685,092 to Dumont, herein incorporated by reference, describes a measurement apparatus wherein a plurality of fixed measurement sensors are directed radially towards the wall of the borehole. These measurement sensors are placed on the apparatus along a helix such that displacing the apparatus parallel to its own axis inside the borehole has the effect of scanning the major portion of the wall of the borehole.
Compared with Broding, Dumont does not provide a complete azimuth coverage of the wall of the borehole. Dumont does, however, teach a calibration means comprising a calibration sensor which is distinct from the measurement sensors and which points downwards, along the longitudinal axis of the apparatus, towards a calibrating target facing this sensor.
By providing substantially the same distance between the target and the calibration sensor as occurs between the measurement sensors and the wall of the borehole, the sensor can be periodically recalibrated while the borehole is being scanned.
However, this solution is only partially satisfactory, in particular due to the fact that the calibration sensor is distinct from the measurement sensors. As a result, the calibration does not take account of any dispersion phenomena that may exist between the measurement sensors. In addition, the calibration sensor points in a different direction than the measurement sensors so that the motion of the fluid contained in the borehole relative to the calibration sensor is not representative of the motion of the same fluid relative to the measurement sensors.